Here a very convenient online BMI calculator
Body Mass Index is an indicator of where you stand in terms of weight in regards to you height. Do you have overweight, underweight or are you in the healthy normal range.
It is a simple number that will tell you if you have a healthy weight (18.5-25) , are under-weight (16-18.5), over-weight (25-30) or even obese (>30) or too underweight (be careful of anorexia) (<16).
Beware: if you are a child or a teenager, normal BMI calculation does not apply to you so look at this page to see if the values are fine.